Interferometric device for detection

ABSTRACT

The device comprises a parallel input microguide (2) and output microguide (5) between which a flat guide is arranged forming a transition zone (10). Coupling between the flat guide and the microguides is achieved by an optical tunnel effect. The transition zone comprises at least two zones, a reference zone (10a) and an interaction zone (10b). The interaction zone (10b) is formed by depositing a superstrate whose optical coefficient, or thickness, is sensitive to the medium to be studied. The light beam transmitted by the input microguide to the flat guide is divided into a reference beam passing through the reference zone (10a) and a measurement beam passing through the interaction zone (10b). The measurement and reference beams interfere in the output microguide (5). Depending on the geometrical shape of the reference and interaction zones, the device constitutes a double wave or a multiple wave interferometer. Such a device is used to achieve sensors for applications in physics, chemistry and biology.

The invention relates to a device for detection of a characteristic of amedium by interferometry, manufactured in integrated optics technologyand comprising, on a substrate, an input microguide connected by aninput end to means for emitting a light beam, forming means to form atleast one reference beam and one measurement beam from said light beam,means for making the reference beam and measurement beam interfere andsupply interference signals, means for detection connected to an outputend of an output microguide transmitting the interference signals, andan interaction zone between the measurement beam and the medium to bestudied.

It is known to use an optic interferometer to detect the presence orconcentration of a gas. A known device using an interferometer of theMach-Zehnder type, achieved in integrated optics technology, isrepresented in FIG. 1. The interferometer comprises a substrate 1 onwhich an input microguide 2 is achieved. The input microguide 2 isdivided into two arms by a first Y-junction, a reference arm 3 and ameasurement arm 4. The two arms are joined, by means of a secondY-junction, in an output microguide 5. An interaction zone 6 with thegas to be studied covers a part of the measurement arm 4. Thisinteraction zone is for example covered by a film able to absorb apredetermined gas and whose refraction coefficient changes according tothe quantity of gas absorbed. An input light beam 7 applied to the inputof the microguide 2 is therefore separated into two beams one of whichis transmitted by the reference arm to the output microguide and theother of which, transmitted by the measurement arm, undergoes a variablephase displacement according to the variation of the coefficient of theabsorbent film. The reference and measurement beams interfere in thesecond Y-junction and the interference signal thus formed detected atthe output of the microguide 5 is representative of the gas to bestudied.

The object of the invention is to achieve a device presenting anincreased sensitivity while being easy to achieve in integrated optics.

According to the invention, this object is achieved by the fact that theforming means comprise a flat guide arranged between the input andoutput microguides in such a way as to achieve light coupling by opticaltunnel effect between each microguide and the flat guide, and comprisingat least a first zone, not sensitive to the medium, and a second zoneconstituting the interaction zone with the medium. The different zonescan be of any shape.

The flat guide is bounded by two sides respectively parallel to theinput and output microguides, each of said sides being arranged near toan intermediate part of an associated microguide.

According to a development of the invention, the interaction zonecomprises a superstrate sensitive to the medium to be studied anddeposited on the substrate.

According to a first alternative embodiment, the input and outputmicroguides are parallel and the interaction zone has the shape of aparallelogram having two sides parallel to the microguides.

According to a second alternative embodiment, the interaction zone hasthe shape of a triangle having one side on the same side as the flatguide associated to the input microguide and an opposite peak on thesame side as the flat guide associated to the output microguide.

Other advantages and features of the invention will become more clearlyapparent from the following description of particular embodiments of theinvention, given as non-restrictive examples only and represented in theaccompanying drawings in which:

FIG. 1 represents a device of known type;

FIG. 2 represents an embodiment of the device according to theinvention;

FIGS. 3 and 4 represent two particular embodiments of the flat guide ofthe device according to FIG. 2, in a double wave interferometer;

FIG. 5 illustrates the amplitude-phase displacement response curve ofthe devices according to FIGS. 3 and 4;

FIGS. 6, 7 and 9 represent three particular embodiments of the flatguide of the device according to FIG. 2, in a multiple waveinterferometer;

FIGS. 8 and 10 illustrate respectively the amplitude-phase displacementresponse curves of the devices according to FIGS. 7 and 9.

The device according to FIG. 2 is achieved in integrated opticstechnology on a substrate 1. As in known devices, an input end of aninput microguide 2 is connected to a light source 8. Such a source canbe formed by a laser diode fixed directly onto the substrate 1 or beconnected to the input of the microguide 2 by means of an optical fibre.The output end of the output microguide 5 is connected to aphotodetector 9. The microguides 2 and 5 are preferably appreciablyparallel and a transition zone formed by a flat guide 10 is arrangedbetween the microguides 2 and 5 in such a way as to achieve coupling ofthe light by optical tunnel effect, also called frustrated totalreflection, between the input microguide 2 and the flat guide 10 andbetween the flat guide 10 and the output microguide 5.

A first, input, part 2a of the microguide 2, connected to the source 8,serves the purpose of stabilising the fundamental mode of themicroguide. Then, in a second, intermediate, part 2b of the microguide 2located near the transition zone 10, the light is coupled by opticaltunnel effect in the zone 10. The coupling is relatively weak, thusensuring a regular light distribution in the zone 10. The non-coupledpart of the light is output from the device via a third, output, part 2cof the microguide 2. A reference photodetector 11 is connected to theoutput of the microguide 2. The reference signal thus measured can beused in a processing unit to correct the variations in the measurementof the interference signal performed by the photodetector 9 due tovariations of the source 8.

The transition zone 10 is divided into at least two zones 10a and 10b. Afirst zone 10a acts as reference zone. The second zone 10b constitutesan interaction zone with the medium to be studied. It is covered by asuperstrate whose optical coefficient and/or thickness is modified whenit interacts with the above-mentioned medium. This modification givesrise to a modification of the propagation constant of the optical wavein the optical medium constituted by the part of the flat guide locatedbelow the superstrate.

The light which has passed through the zone 10 is coupled by opticaltunnel effect with an intermediate part 5a of the output microguide,located nearby. The part 5a thus forms a coupling and interference zonein which interference signals form constituted by the sum of the lightrays coming from the part 2b and having passed respectively through thezones 10a and 10b. There is therefore interference between the referencelight beams having passed through the reference zone 10a and measurementlight beams, displaced according to the composition of the medium to bestudied. This is particularly illustrated in FIG. 3 where a few lightrays are represented. The interference signals are transmitted to thephotodetector 9 by an output part 5b of the microguide 5.

The reference zone 10a and interaction zone 10b may have any geometricalshape. In FIG. 2, they are separated by a line 10c of totally arbitraryshape. According to a preferred embodiment represented in FIG. 2, thetransition zone 10 is trapezoid. It comprises a small base 12 near tothe part 2b and parallel to the latter and a large base 13 near to thepart 5a of the output microguide 5. In FIG. 2 it has the shape of arectangular trapezoid whose inclined side 14 is arranged on the sameside as the output ends of the microguides 2 and 5.

Such a device is very simple to achieve and easily reproducible. Astandard basic device comprises the microguides 2 and 5 and the flatguide 10 and possibly the source 8 and photodetectors 9 and 11. Such adevice can be manufactured in series and be adapted to suit requirementsby simply depositing a superstrate suited to the medium to be studiedand whose geometrical shape is chosen arbitrarily. The reference zone10a can if required also be covered by any material not sensitive to themedium to be studied.

In the alternative embodiments of FIGS. 3 and 4, the interaction zone10b has the shape of a parallelogram having two sides situatedrespectively on the small base 12 and on the large base 13 of therectangular trapezoid constituting the transition zone. In FIG. 3, theinteraction zone is arranged in the centre part of the zone 10. Thereference zone 10a is then formed by the remainder of the zone 10located above the zone 10b. In FIG. 4, the zone 10b is arranged in theupper part of the zone 10. In both cases the interferometer thus formedis a double wave interferometer whose response curve is of the formrepresented in FIG. 5. The curve represents the intensity | of theinterference signals with respect to the phase displacement .O slashed.,introduced by the interaction zone 10b, between the measurement andreference beams. Such a curve has the following form:

    |=|o (1+Cos .O slashed.)                 (1)

where |o is the intensity of the signals in the absence of phasedisplacement. The phase displacement .O slashed. can be proportional tothe refraction coefficient n of the superstrate deposited on theinteraction zone 10b, variable according to the medium to be studied. Aresponse of this type is suitable for large measurement ranges. Thewidth of the interaction zone 10b with respect to the reference zone 10adecides the distribution of the light intensity between these two zones,which enables the contrast of the interferometer to be optimised.

In the alternative embodiments of FIGS. 6, 7 and 9, the interaction zone10b has the shape of a triangle one side of which is situated on thesmall base 12 of the trapezoid and whose opposite peak 15 is situated onthe large base 13 of the trapezoid. In FIGS. 6 and 7, theabove-mentioned side of the triangle has the same length as the smallbase 12 whereas in FIG. 9 said side is smaller than the small base 12.In FIG. 6, the peak 15 is situated in the middle part of the large base13. In FIGS. 7 and 9, a second side of the triangle is situatedidentically with the inclined side 14 of the trapezoid.

The response curve of the embodiments of FIGS. 6 and 7, represented inFIG. 8, has the form:

    |=|o (1-Cos .O slashed.)/.O slashed..sup.2 (2)

This response is more particularly suited to unambiguous leveldetection, a predetermined intensity level corresponding to a givenphase displacement level, when above the level of the oscillations.

FIG. 10 represents the response curve of the alternative embodiment ofFIG. 9, which is an intermediate response between those of FIGS. 5 and8.

In the three embodiments of FIGS. 6, 7 and 9, the interferometer thusformed is a multiple wave interferometer.

The invention is not limited to the particular geometrical shapesrepresented in the figures but extends to any geometrical shape of thetransition zone and of the lines separating the interaction zone 10b andthe reference zone 10a. The zone 10 can in particular be divided intoseveral sensitive zones and into several reference zones.

The amplitude-phase displacement response of the interferometer istherefore determined by the geometrical shape of the layers deposited onthe substrate of the device. It can be of the same type as that of aresonator, of a double wave interferometer, or of any other intermediateresponse.

In a double wave interferometer of known type, as represented in FIG. 1,depositing an absorbent substrate on the interaction zone 6 mayunbalance the arms of the interferometer, which has a harmful effect onthe contrast of the measurement signal. With a device according to theinvention, free choice of the interaction zone geometry enables thisproblem to be avoided.

Moreover, the interaction surface 10b may be large, which reduces thesensitivity of the interferometer to any possible non-homogeneities ofthe substrate covering the interaction zone.

Furthermore, the device according to the invention does not, unlikeknown Fabry-Perot type interferometers, comprise reflecting elementswhich are delicate to achieve in integrated optics. Neither does itcomprise curved guides, unlike the known device according to FIG. 1,which guides cause problems in certain integrated optics technologies.This results in a simplicity of manufacture which can lead to asubstantial reduction of the production cost of a sensor of this type.

The device naturally also presents all the advantages inherent tointegrated optics, notably insensitivity to vibrations, compactness,easy temperature control, possibility of manufacture in series andresistance to electromagnetic disturbances.

The device described above can be used to achieve sensors forapplications in physics, chemistry or biology, the medium to be studiedcoming into contact with the superstrate deposited on the interactionzone. More generally it can be used each time an interaction is to beestablished between an external medium and light.

We claim:
 1. A device for detection of a characteristic of a medium byinterferometry, manufactured in integrated optics technology andcomprising, on a substrate (1), an input microguide (2) connected by aninput end to means (8) for emitting a light beam, forming means (10a,10b) to form at least one reference beam and one measurement beam fromsaid light beam, means (5a) for making the reference beam andmeasurement beam interfere and supply interference signals, means (9)for detection connected to an output end of an output microguide (5)transmitting the interference signals, and an interaction zone (10b)between the measurement beam and the medium to be studied, a devicecharacterized in that the forming means comprise a flat guide (10)arranged between the input and output microguides (2, 5) in such a wayas to achieve light coupling by optical tunnel effect between eachmicroguide and the flat guide, and comprising at least a first zone(10a), not sensitive to the medium, and a second zone (10b) constitutingthe interaction zone with the medium.
 2. The device according to claim1, characterized in that the interaction zone comprises a superstratesensitive to the medium to be studied and deposited on the substrate(1).
 3. The device according to claim 2, characterized in that theoptical coefficient of the superstrate varies according to thecharacteristic of the medium to be studied.
 4. The device according toclaim 2, characterized in that the thickness of the superstrate variesaccording to the characteristic of the medium to be studied.
 5. Thedevice according to claim 1, characterized in that the flat guide (10)is bounded by two sides (12, 13) respectively parallel to the input andoutput microguides (2, 5), each of said sides being arranged near to anintermediate part (2b, 5a) of an associated microguide.
 6. The deviceaccording to claim 5, characterized in that the side (13) of the flatguide associated to the output microguide (5) is longer than the side(12) of the flat guide associated to the input microguide (2).
 7. Thedevice according to claim 5, characterized in that the input and outputmicroguides (2, 5) are parallel and that the interaction zone (10b) hasthe shape of a parallelogram having two sides parallel to themicroguides (2, 5).
 8. The device according to claim 5, characterized inthat the interaction zone (10b) has the shape of a triangle having oneside on the same side (12) as the flat guide (10) associated to theinput microguide (2) and an opposite peak (15) on the same side (13) asthe flat guide (10) associated to the output microguide (5).
 9. Thedevice according to claim 1, characterized in that it comprises means(11) for detecting a reference light signal connected to an output endof the input microguide (2).